Graphite calorimetry as a primary dosimetry method. The experience of ENEA-INMRI in radiotherapy dosimetry

A talk delivered by invitation at ELETTRA Sincrotrone Trieste

Transcript

1. Graphite calorimetry as a primary dosimetry method. The experience of ENEA-INMRI in radiotherapy dosimetry Massimo Pinto Sezione di Dosimetria Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti Centro Ricerche ENEA Casaccia, Roma massimo.pinto@enea.it ELETTRA Sincrotrone Trieste Wednesday, December 20th 2017 1
2. Measuring energy per unit mass Measuring kerma or dose requires a measurement of energy imparted to a given material, and a measurement of its mass. For the measurement of dose, the determination of energy with a ‘primary’ method offers three routes: 1.  Ionometry 2.  Calorimetry 3.  Fricke dosimetry Seuntjens J, Duane S. Photon absorbed dose standards, Metrologia, 2009;46(2):S39–S58.
3. Measuring energy per unit mass Dose rate Ionometry Calorimetry Fricke Dosimetry The intensity of the signal can influence the method of choice
4. Calorimetry has been used for several decades as a technique to measure absorbed (radiation) dose to water. The underlying assumption is that all (or a known fraction, due to a phenomenon called heat defect) of the absorbed radiation energy appears as heat, so that the measurement of absorbed dose reduces to a measurement of a temperature variation. Which will depend on the specific heat capacity of the medium. Graphite vs water calorimetry temperature variation after 1 Gy (10-3 K) 1.4 0.24
5. Accuracy constraints Medium material temperature variation after a dose of 1 Gy (10-3 K) target undertainty minimum temperature variation that needs to be appreciated (10-3 K) graphite 1.4 0.01 0.014 water 0.24 0.01 0.002
6. Water calorimeters Dw Graphite calorimeters Dcal Dw Graphite better than water? experimental measurements, some corrections experimental measurements, some corrections more corrections (usually via Monte Carlo)
7. Pros and Cons Medium material Pros Cons graphite Sensitivity, Good control of design, mechanical realization and thermal insulation Monte Carlo simulations needed to convert Dcal to DW water (Almost) direct realization of the quantity of interest Heat defect, Relatively insensitive
8. DW calorimetric standards at ENEA Absorbed dose to water primary standard for 60Co gamma radiation (-’90) Absorbed dose to water primary standard for dosimetry in brachytherapy with high-dose rate 192Ir sources (2012) Absorbed dose to water primary standard in medium-energy x-ray filtered beams (2016)
9. Graphite calorimeters for the measurement of the absorbed dose to water Antonio Guerra, Marco D’Arienzo
10. The primary standard of absorbed dose to water established at ENEA for the 60Co gamma-ray quality is based on a graphite, Domen-type calorimeter and an ionometric transfer system. The calorimeter is thermo- regulated at a temperature of about 27°C, with a stability better than 5 ·10-4 °C during a typical measurement run. Absorbed dose to water primary standard for 60Co gamma radiation Antonio Guerra, Marco D’Arienzo
11. Absorbed dose to water primary standard for 60Co gamma radiation The experimental assembly which constitutes the italian standard for Dw for 60Co is a graphite calorimeter of the Domen type, a graphite phantom, a water phantom and a thick-walled (TW) graphite ionization chamber. Graphite calorimeter TW chamber into the graphite phantom Antonio Guerra, Marco D’Arienzo
12. Absorbed dose to water primary standard for 60Co gamma radiation Antonio Guerra, Marco D’Arienzo
13. Current configuration (60Co calorimeter) Antonio Guerra, Marco D’Arienzo
14. Absorbed dose to water primary standard for dosimetry in brachytherapy with high dose rate 192Ir sources Marco D’Arienzo
15. Absorbed dose to water primary standard for dosimetry in brachytherapy with high dose rate 192Ir sources Marco D’Arienzo
16. Detail of a sensing thermistor embedded in graphite. Absorbed dose to water primary standard for dosimetry in brachytherapy with high dose rate 192Ir sources Marco D’Arienzo
17. Absorbed dose to water primary standard for dosimetry in brachytherapy with high dose rate 192Ir sources Marco D’Arienzo
18. The source catheter inserted in the calorimeter (MicroSelectron v2 HDR 192Ir source) Absorbed dose to water primary standard for dosimetry in brachytherapy with high dose rate 192Ir sources Marco D’Arienzo
19. Absorbed dose to water primary standard in medium-energy x-ray filtered beams
20. Motivation for establishing a new Dw standard at ENEA-INMRI The IAEA TRS 398 dosimetry protocol calls for traceability to absorbed dose to water primary standards in Dw calibrations for all modalities. The application of a simpler protocol reduces the risk of errors. now under revision! Due 2019
21. “…The various steps between the calibration of ionization chambers in terms of the quantity air kerma, Kair , at the standardizing dosimetry laboratories and the determination of absorbed dose to water, Dw , at hospitals […] introduce undesirable uncertainties into the realization of Dw .” Motivation for establishing a new Dw standard at ENEA-INMRI
22. “...propose the calibration of therapy level dosimeters in terms of Dw , stressing the advantages of using the same quantity and experimental conditions as the user” Motivation for establishing a new Dw standard at ENEA-INMRI
23. Three water calorimeters existing in Europe and (almost) ready to use for med-energy x-rays, but graphite calorimetry for med-energy x-rays never reported before. To compare it to water calorimeters and therefore consolidate this specific area of radiation dosimetry. Funded by the European Union within the EMRP “MetrExt” Joint Research Project 1 1 Motivation for establishing a new Dw standard at ENEA-INMRI
24. Calorimeter construction ! Calorimeter core sitting on the jacket base element Shield base and jacket base elements Built thanks to funding from the EU project
25. Fixing a thermistor in place
27. Operation – CCRI 180 example
28. Data analysis – CCRI 180 example
29. Conversion of absorbed dose from graphite to water Dw in homogeneous water absorbed dose in the calorimeter graphite core, Dcal w,g cal w C D D = Cw,g determined by Monte Carlo calculations Calculations made by Maria Pimpinella using egsNRC
30. Data analysis needs speeding up! 30 courtesy of Claus Andersen, DTU Denmark
31. 31 Data analysis needs speeding up! courtesy of Claus Andersen, DTU Denmark
32. Automation gets essential 32 courtesy of Claus Andersen, DTU Denmark
33. Uncertainty Budget Uncertainty component uA uB fc , fractional change of thermistor resistance during irradiation / (ΔΩ/Ω) C-1) 0.015 0.0014 kqa , quasi adiabatic calibration factor / (J/( ΔΩ/Ω)) 0.0016 0.0021 kr , radial non-uniformity of the beam across the core surface 0.0014 0.0010 Cw,g , graphite to water absorbed dose conversion factor 0.0010 0.0093 mc , effective core mass 0.0010 0.0046 quadratic sum 0.0152 0.0112 combined standard uncertainty 0.019
34. New setup (late 2015)
35. EURAMET Comparison RI(I)-S13 To carry out a comparison of primary standards of Dw in the medium-energy x-ray range. Institute Country Standard PTB Germany Water calorimeter LNE-LNHB France Water calorimeter VSL B.V. The Netherlands Water calorimeter MKEH Hungary Extrapolation Chamber ENEA-INMRI Italy Graphite calorimeter
36. Uncertainty Budget getting better Uncertainty component uA uB fc , fractional change of thermistor resistance during irradiation / (ΔΩ/Ω) C-1) 0.015 -> 0.006 0.0014 kqa , quasi adiabatic calibration factor / (J/( ΔΩ/Ω)) 0.0016 0.0021 kr , radial non-uniformity of the beam across the core surface 0.0014 0.0010 Cw,g , graphite to water absorbed dose conversion factor 0.0010 0.0093 -> mc , effective core mass 0.0010 0.0046 -> quadratic sum 0.0152 0.0112 combined standard uncertainty 0.019 -> ?
37. Simpler solutions for higher dose rates Great care is needed in the design and construction of a graphite calorimeter when the relatively low dose rates demand high thermal insulation. But what about when the signal is really abundant? The Australian experience on a synchrotron beam 37
38. A simpler graphite calorimeter 38 Harty PD, Lye JE, Ramanathan G, Butler DJ, Hall CJ, Stevenson AW, et al. Absolute x-ray dosimetry on a synchrotron medical beam line with a graphite calorimeter. Medical Physics 2014, May; 41(5):052101.
39. A simpler graphite calorimeter A much simpler layout. Abundance of signal simplifies design in that lower care is put to the thermal insulation of the calorimeter core from the other bodies of the instrument (when they exist). The number of bodies is reduced! 39 Harty PD, Lye JE, Ramanathan G, Butler DJ, Hall CJ, Stevenson AW, et al. Absolute x-ray dosimetry on a synchrotron medical beam line with a graphite calorimeter. Medical Physics 2014, May; 41(5):052101.
40. A simpler graphite calorimeter High dose rates make it for a signal-to-noise ratio and minimizes the perturbations induced to the measurements by heat exchanges across the calorimeter bodies and from the calorimeter towards the outside environment. 40 Harty PD, Lye JE, Ramanathan G, Butler DJ, Hall CJ, Stevenson AW, et al. Absolute x-ray dosimetry on a synchrotron medical beam line with a graphite calorimeter. Medical Physics 2014, May;41(5):052101.
41. Calorimetry on synchrotron beams Lye JE, Harty PD, Butler DJ, Crosbie JC, Livingstone J, Poole CM, et al. Absolute dosimetry on a dynamicallyscanned sample for synchrotron radiotherapy using graphite calorimetry and ionization chambers. Phys Med Biol. IOP Publishing; 2016 May 17;61(11):4201–22. 41
42. Almost fifty years of calorimetry Calorimetry for dosimetry of ionizing radiation started late in the ‘60 from the ambition of, probably, a single visionary man. It’s another way to look at dosimetry and adds to the fun. 42
43. Thank you for another wonderful week at ELETTRA. 43